Improving Fertilizer Guidelines for California's Changing Rice Climate, 2020

 

Bruce Linquist, UCCE Rice Specialist, Dept of Plant Sciences, UC Davis

The overall goal of this project is to develop fertilizer management guidelines that are economically viable and environmentally sound. Research objectives in 2020 were:

• Assess top-dressed nitrogen needs through remote sensing.

• Study potential nitrogen losses early in the season due to flooding and draining events.

• Develop alternative nitrogen fertilizer management strategies.

Remote sensing of nitrogen status

This study was initiated in 2015 to evaluate the potential for sensor-based technologies to assess nitrogen status in rice and to determine the need for a top-dressed nitrogen application.

A handheld sensor, the Green Seeker NDVI, was used to evaluate multiple farmer fields. NDVI is an acronym for “normalized difference vegetation index” and it has shown to be a strong predictor of crop nitrogen uptake and final grain yield.

Scientists provide more detail on their research at a Rice Field Day

More recently, a drone was incorporated into the study. It demonstrated that another remote sensing metric called NDRE (Normalized Difference Red Edge) also is a strong predictor of crop nitrogen uptake.

In 2020, researchers focused on refining both response indexes with a more advanced analysis. Previous analysis showed that a response index value of 1.10 was the threshold that a top-dressed fertilizer application would provide a yield response. Results from 2020 refined this value to 1.07 or greater. When a top-dressed nitrogen application was needed and applied, yields increased by roughly 200 pounds/acre to 1,000 pounds/acre. The larger the response index, the greater the potential yield increase.

These are encouraging results that provide a more robust tool to accurately assess a rice field during panicle initiation to determine whether the top-dress will produce a yield increase.

Potential early season nitrogen loss

In conventionally managed rice, fields are flooded shortly after aqua nitrogen is applied. Flooding protects the aqua nitrogen fertilizer from converting to nitrate because the field is in an anaerobic state. This results in good nitrogen use efficiency.

However, when a field is drained that still has aqua nitrogen in the soil, oxygen is introduced and microbes can then convert the ammonium to nitrate through a process called nitrification. Nitrate, in turn, can then denitrify when the field is reflooded and is lost as nitrogen fertilizer. The process is driven by how aerobic (dry) soils get during a dry down period, soil temperature, and soil properties such as texture and carbon content. With knowledge of how fast the conversion of ammonia to nitrate takes place under different soils and drying conditions, growers will have a better idea of how much nitrogen is still available in the soil.

In 2019, two field studies were conducted—one at the Rice Experiment Station (RES) and one near Knights Landing. In 2020, one study was conducted at RES in fields that were either in continuous flood or drained twice, each with and without a preplant aqua ammonia application. The drained treatments are typical of how a dry-seeded field is managed.

Based on data from both years of this study, fertilizer nitrogen losses from draining and reflooding dry-seeded rice fields resulted in a nitrogen fertilizer loss of roughly 20 to 30 pounds an acre. The losses are primarily due to denitrification but also a small amount from nitrous oxide emission into the atmosphere (about one to two pounds an acre).

Alternative nitrogen management

Most rice grown in California rice receives the majority of nitrogen fertilizer in the form of aqua ammonia. Some top-dressed nitrogen is applied before planting or delayed by a few weeks, a strategy that is usually efficient and effective. However, in 2019 late season rain forced many growers to plant their rice before any fertilizer was applied. This resulted in a situation where growers tried a wide variety of approaches to applying nitrogen, some successful and some not.

Since no research had been done in this area, growers had no recommendations to go on. Given that uncertainty, an experiment was conducted comparing aqua ammonia, ammonium sulfate, and urea applied as the nitrogen source. The study looked at different timings of applications, splitting applications and the use of specialty urea products that delay or slow the release of nitrogen. The study was conducted at the Rice Experiment Station, with 15 treatments all receiving 150 pounds/acre nitrogen. The treatments were designed to examine different sources, as well as timing of fertilizer. Timing was preplant, immediately after planting, and split applications.

Some general findings can be made about the post-planting fertilizer applications. All fertilizers increased grain yields relative to the control. Applying granular fertilizer in a single dose two weeks after planting was better than applying it all immediately after planting. Split applications resulted in the highest yields. This study will be repeated in 2021.

Other experiments

Lodging in M-105, M-206

A trial was conducted in 2020 at the Rice Experiment Station to evaluate lodging in M-105 and M-206. Growers have become concerned about lodging in M-105 and in some cases have stopped growing it.

Nitrogen rates were 120, 150, 180, and 210 pounds nitrogen/acre, with subplots of top-dressed nitrogen at 30 pounds/acre during panicle initiation. The experiment found M-206 more prone to lodging. Lodging increased with increased nitrogen rates. Lodging was most severe with top-dressed nitrogen.

Yields were not significantly different across nitrogen rates or between varieties. Yields were highest at 120 pounds/acre nitrogen. At higher nitrogen rates, yields tended to decline.

Planting date

A planting date trial was initiated in 2020 for the primary medium grains grown in California. Planting dates were May 29, June 12, and June 26. See the charts below.

Other studies have shown that a two-week delay in planting results in a week delay in heading. M-105 headed the earliest (73 to 76 days) and M-211 headed the latest (80 to 83 days).

Highest yields were obtained from the June 12 planting. M-211 had the highest yields in the first two planting dates.

A medium grain trial showed highest overall yields from the June 12 planting date.
Time to maturity did not vary much in this planting date trial. Other studies have shown a two-week delay in planting can result in a one-week delay in heading.

Seed treatment product

A trial at the Rice Experiment Station evaluated a seed treatment product called Envita® to see whether it would increase nitrogen uptake and yields. This product is designed to stimulate nitrogen fixation, increasing uptake and thus lowering fertilizer nitrogen requirements.

The variety used in this experiment was M-206. Soaked seeds were planted June 1. Envita® was evaluated at different nitrogen rates. Researchers did not see a yield benefit to using the product.